A primary requirement in the optical industry in regard to apparatus for polishing ophthalmic lenses, is the ability to improve the surface finish of a lens without changing the curvature of the lens' surfaces. Ideally, a lens polishing operation is limited to the removing of minor scratches and machining marks in a lens while maintaining perfectly the prescribed curvatures. It will be appreciated that any uncontrolled pressure point in a lapping membrane of a lapping tool head, or a depletion of polishing slurry at any location over the membrane can cause the polishing operation to remove more or less material in one region of the lens, thereby changing the prescribed curvature of that lens and causing optical defects.
Many types of lens polishing apparatus have been developed in the past and were used with varying degrees of success. These apparatus are believed to belong to two broad groups. The first group utilizes a lapping tool head having a resilient or flexible lapping membrane which is deformable upon contact with the surface of a lens to adapt to the curvature of the lens. Examples of apparatus belonging to this first group are described in the following U.S. Patents: U.S. Pat. No. 3,589,071 issued on Jun. 29, 1971 to Hans S. Hirschhorn; U.S. Pat. No. 5,205,083 issued on Apr. 27, 1993 to Dennis R. Pettibone; U.S. Pat. No. 5,662,518 issued on Sep. 2, 1997 to Michael D. James et al.
The second type of lens polishing apparatus of the prior art uses a plurality of plungers for applying pressure gradients over a lens polishing membrane. Although these apparatus are designed for polishing large telescope mirrors, this is the type of apparatus that is of interest herein. Examples of these apparatus are illustrated in the following U.S. Patents: U.S. Pat. No. 4,606,151 issued on Aug. 19, 1986 to Erich Heynacher; U.S. Pat. No. 4,802,309 issued on Feb. 7, 1989 to Erich Heynacher; U.S. Pat. No. 4,850,152 issued on Jul. 25, 1989 to Erich Heynacher et al.
The latter examples describe polishing apparatus having a plurality of actuators for applying different pressures at different areas of a polishing membrane. The different pressures are adjusted according to the amount of material to be removed at different locations on the lens surface, such that a polishing operation is effected more quickly. These apparatus have undeniable merits and are believed to be great advances in the polishing of optical surfaces.
However, these apparatus do not address the fact that a lens polishing operation is normally accompanied by the accumulation or depletion of abrasive slurry at certain regions of the lens surface, which can cause more or less polishing in these regions. Although this is a very common source of optical defects in ophthalmic lenses, there is no known prior art apparatus that offers a solution to the controlling of the movement of abrasive slurry between a polishing membrane and a lens surface.